Mouse cells contain two distinct activities for thymidine kinase. The mitochondrial associated enzyme is most prominent in resting cells while the cytoplasmic activity is found exclusively in growing cells in culture. Resting cells infected with SV40 or adenovirus specifically stimulate the cytoplasmic activity. SV40 tsA or D mutants fail to stimulate this enzyme when an infection is carried out at the nonpermissive temperature. Temperature shift experiments will be carried out to see if the A gene product is required transiently or continually for the stimulation of this enzyme activity. Mutants of adenovirus will be tested for their ability to stimulate this enzyme after infection. It is hoped that these studies will be useful in elucidating the mechanism by which these viruses can turn on cellular gene activities.